The invention relates to a system for visually indicating points or shapes on a structure, such as an airframe for an aircraft or the like, at which work operations are to be performed. Such work operations may include attaching a piece of hardware to the structure at the indicated point, marking the indicated point with letters or graphical information for later use by workers, or other operations. The invention relates more particularly to a system employing a laser head that projects a laser beam onto the work locations on a structure at which work operations are to be performed so that workers can mark or attach hardware at the indicated work locations.
The assembly of an aircraft structure can be a complicated operation involving hundreds or thousands of individual work operations for attaching various pieces of hardware to the structure. It is common practice to mark the various members of an aircraft structure with markings such as lettering, stencils, or the like, so as to indicate to assembly workers the locations on the structure where various work operations are to be performed and to indicate what those work operations entail. It is generally necessary to apply the markings after the main structure is assembled, in part because it is common to paint the structure with a protective paint after it is assembled, which would obliterate any markings applied prior to assembly. Furthermore, it is advantageous for reasons related to tolerance stack-up in many cases to wait until after a structure is assembled to mark the locations of hardware attachment points or additional secondary structure. The problem then becomes how to find those locations once the structure is assembled.
Engineering drawings, whether in conventional hardcopy or digital electronic form, are difficult to generate with sufficient simplicity and dimensional detail to make them easy to interpret for the workers responsible for applying the markings or attaching the hardware to the structure. To aid workers in applying the markings and attaching hardware, laser projection systems have been developed. In a complicated structure, and particularly in a closed interior structure such as an airframe, the laser may have to be relocated numerous times in order to have a clear line of sight to all of the points on the structure at which work operations are to be performed. Each time the laser is relocated, the support for the laser must be located in a known fashion relative to the structure and the laser must be calibrated to known reference locations on the structure. The numerous set-ups would be so time-consuming on a complex structure that the use of the laser projection system would be prohibitive in cost.
The present invention addresses the above needs by providing a programmable traversing laser locating system that enables the laser to be easily relocated in a known, repeatable fashion to as many locations as required to establish clear lines of sight to all of the work locations on the structure. The system employs a laser head that travels along a track through, into, or about the structure, and a drive system operable to move the laser head along the track. The laser head includes an actuator operable to point the laser beam generated by the laser head in various directions. The drive system and actuator provide signals indicative of the position of the laser head along the track and the orientation of the laser beam, respectively, enabling the drive system and actuator to be controlled so as to position the laser head in a desired position along the track and to point the laser beam in a desired direction.
A method in accordance with a preferred embodiment of the invention includes the step of predetermining, relative to a reference coordinate system, coordinates of the work locations of the structure onto which the laser beam is to be projected and coordinates of at least three reference locations on the structure. The reference coordinate system may be, for example, an XYZ coordinate system in which a digital representation of the structure has been created, such as in a CAD-CAM system. The digital representation may include the coordinates of the reference locations and the coordinates of the work locations at which work operations are to be performed. The method further includes the step of erecting an elongate track such that the track is fixed in space and passes proximate to the structure, and mounting a laser head on the track such that the laser head is traversable along the track. The laser head is operable to emit a visible laser beam and includes an actuator for varying the direction of the laser beam. The laser head is coupled with a drive system for traversing the laser head along the track, the drive system providing feedback signals indicative of the position of the laser head along the track and the actuator providing feedback signals indicative of the direction of the laser beam relative to an internal coordinate system of the laser head. To orient the laser head relative to the structure, the drive system is operated to traverse the laser head to a position (or to more than one position) along the track having a clear line of sight to each of the reference locations and the actuator of the laser head is operated to direct the laser beam onto each of the reference locations. A distance from each reference location to the laser head is measured. Since the coordinates of the reference locations are known in the reference coordinate system, it is possible to calibrate the drive system and the actuator based on the feedback signals therefrom and the measured distances such that coordinates of the laser head and direction of the laser beam can be determined in the reference coordinate system from the feedback signals from the drive system and actuator. Once this calibration is performed, the laser beam can be pointed to any desired point in space in the reference coordinate system. Thus, the drive system and the actuator are operated to direct the laser beam onto each of the work locations on the structure.
As noted, calibrating the drive system and actuator involves measuring a distance from the laser head to each of the reference locations of the structure. Based on the measured distances and the direction of the laser beam as indicated by the signals from the actuator, coordinates of the reference locations are calculated in the coordinate system of the laser head. In general, the calculated coordinates will differ from the predetermined coordinates of the reference locations in the reference coordinate system. Thus, a transformation (e.g., a least-squares fit) between the laser head""s coordinate system and the reference coordinate system is determined, enabling the laser beam to be projected onto any point on the structure whose coordinates in the reference coordinate system are known.
In one embodiment, the laser head can be a laser tracker having the capability of using laser interferometry to determine the distance to each reference location. Such laser heads project an incident laser beam onto a retroreflective target, which reflects the beam back to the laser head where it is detected by a receiver. Comparison of the incident and reflected beams enables the distance to the target to be calculated. Accordingly, where such a laser head is employed, retroreflective targets are attached to each of the reference locations on the structure during the calibration phase of the method. The laser head if desired can be a known type of laser tracker operable to automatically xe2x80x9cacquirexe2x80x9d each of the targets by executing a routine in which the laser beam is systematically directed in different directions, such as in progressively smaller and smaller circles, until the laser head xe2x80x9clocks ontoxe2x80x9d the target. The laser head also can be an oscillating type operable to trace a line, curve, or perimeter on the structure. This can be useful for tracing the outline of a bracket or other piece of hardware to be attached to the structure.
The track along which the laser head is traversed and the drive system for moving the laser head can be an integral linear positioning system having a beam-like housing on which a support plate is slidably mounted, and a drive belt mounted within the housing and attached to the support plate. The laser head is mounted on the support plate. The drive belt can be driven by a servo motor providing position feedback. Of course, other types of tracks and drive systems can be employed if desired.
Preferably, the method of the invention entails predetermining positions along the track at which the laser head has a clear line of sight to reference locations and work locations on the structure. Depending on the structure, the laser head may have to be stopped in several different positions to be able to see all of the reference locations and work locations. It may also be necessary to provide more than one track each carrying a laser head. It is also desirable for the laser beam to be as close to normal to the surface of the structure as possible so that the beam spot on the structure will have good definition. Thus, the predetermined positions along the (or each) track are selected such that the laser head has a clear line of sight to one or more locations on the structure onto which the laser beam is to be projected, and such that the laser beam can be projected onto one or more work locations with an acceptably small angle between the beam and the surface normal at each work location. In many cases, a deviation of as much as 75 degrees between the beam and the surface normal can be tolerated.
Preferably, the coordinates of the reference locations and work locations are stored in an electronic storage medium, such as a hard disk, that is connected to a programmed microprocessor controller. The controller is also connected to the drive system and actuator for the laser head. The controller is programmed to operate the drive system and actuator so as to position the laser head in the corresponding position along the track for each reference and work location. Accordingly, the data stored in the storage medium preferably also includes the track position for each reference and work location. It is also advantageous for the stored data to include information about the work operation to be performed at each work location, such as a description of the marking to be applied to the structure (e.g., lettering or graphical indicia), the size, font, and color to be used for any lettering, the orientation of the marking, an identification of hardware to be attached at the work location, and/or other information considered useful. The storage medium can be a network server located remote from the structure or a dedicated storage device located where the structure is erected.
The controller preferably is also operable to calculate the transformation between the internal coordinate system of the laser head and the reference coordinate system for the structure. It is advantageous for the controller to identify any reference point whose calculated coordinates differ from the predetermined coordinates by more than a selected tolerance limit, since this may indicate a problem with the structure. The controller can also give the operator an option to disregard any such out-of-tolerance reference point(s) when calculating the transformation.
It is further preferable to connect the controller with one or more display devices, such as a CRT monitor, LED display, heads-up display, or the like, so that the information for each work location can be visually displayed. Prompts for the operator can also be displayed.
The system preferably includes an input device for the controller so that the operator can provide responses to prompts from the controller and can input information for storage in a log file on the storage medium. The input device can be, for example, a wireless remote control device in communication with the controller by infrared signals or the like and operable to make selections from a predefined user menu displayed on the display device. The menu can include selections such as xe2x80x9cgo to next pointxe2x80x9d (meaning that the work operation for the current point was successfully completed and the laser head is to illuminate the next work location in the stored list of predetermined work locations), xe2x80x9cgo to previous pointxe2x80x9d, xe2x80x9cskip to next pointxe2x80x9d (meaning that the work operation for the current point could not be performed for some reason and the laser head should skip to the next point in the list), xe2x80x9cre-acquire reference pointsxe2x80x9d (causing the laser head to automatically re-acquire the reference points and the controller to calculate a new transformation), xe2x80x9cauto-inspectxe2x80x9d (causing the laser head to toggle at a predefined rate through all of the work locations for the current position of the laser head), and/or other selections. The controller preferably flags in the log file each work location for which the work operation was not performed, so that workers responsible for subsequent assembly operations will readily be able to identify those locations by reviewing the log file.